In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. In the page-oriented holographic storage approach, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. Data pages can also be produced by an array of phase shifting elements. The use of data pages allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
To make efficient use of the holographic storage material, a good overlap between the object beam and the reference beam is necessary. A good overlap with the object beam means that the entire or at least a significant part of the object beam is superimposed by the reference beam inside the holographic storage medium. In order to improve this overlap, it has been proposed to use two or more reference beams during recording or reading.
For example, EP 1 624 451 discloses a coaxial holographic storage system, where a plurality of reference beams are arranged around the object beam. According to this solution the object beam and the reference beams are coupled in and out at the object plane and the image plane, respectively. This arrangement is a so-called split aperture arrangement, because the aperture of the Fourier objective is split into an object part and a reference part.
Though the use of multiple reference beams allows to improve the overlap with the object beam, it has been found that during recording it becomes necessary to reduce the intensity of the reference beams to avoid local saturation of the holographic material. This leads to an effective reduction of the M# number, or in other words a reduction of the number of holograms that can be stored in the holographic storage medium.